Microencapsulation Product and Process

ABSTRACT

Microcapsules possessing Lewis acid-Lewis base salt walls incorporate water-immiscible materials, such as N,N-diethyl-m-toluamide (DEET), as a core component. Such microcapsules, or similar microcapsules incorporating other core components, may be made by emulsifying a water-immiscible core component in an aqueous solution of one wall-forming reactant, such as the Lewis base, and then mixing that solution with the other wall-forming reactant, such as the Lewis acid. Various adjuvants may be included with the core component to contribute additional characteristics, such as enhancement of a controlled release characteristic or improved mechanical stability.

FIELD OF THE INVENTION

This invention relates generally to the encapsulation and controlledrelease of materials and to related encapsulated products and processesfor making and using such products. Because capsules thus formed aregenerally of very small size, these products and processes are oftenreferred to as microcapsules and microencapsulation processes.

BACKGROUND OF THE INVENTION

The use of droplets or particles of an active substance incorporated inan inactive carrier or carrier composition is well known in various artsincluding pharmaceutical, medical, agricultural, and many others.Typically, specific compositions provide for the application of specificactive substances in a quantity or concentration appropriate to the use,and are particularly well suited to cases where the active substanceitself is not easily compounded into a suitable vehicle or to facilitatecontrolled release of the active substance.

Encapsulation methods and materials are diverse and are well known tothose skilled in the art of preparing controlled-release formulations.Encapsulation may take the form of an enclosing wall of inactive agentaround a solid or liquid core of active agent, or it may take the formof a continuous matrix of porous inactive agent that contains the activeagent in the manner of a sponge or foam.

Among known encapsulation compositions and methods are those based onthe formation of capsular walls by the reaction of a Lewis acid and aLewis base, aligned at a droplet interface in an emulsified two-phase(generally aqueous-organic solvent) mixture, with a core materialtrapped in the droplet to be encapsulated. A number of such compositionsand methods, and variants thereof, are disclosed in U.S. Pat. Nos.3,959,457, 4,743,583, 4,797,234, 4,917,892, 5,093,198, 5,132,117,5,284,663, 5,490,986, 5,686,113, 6,270,800, and 6,531,156, in all ofwhich Dr. Tully Speaker is the inventor or a co-inventor and in some ofwhich the present inventor is a co-inventor.

Other encapsulation methods are also known in the art. In general, manyknown methods have characteristics that may in some cases make themsomewhat inconvenient to practice, such as a need for ultrasonicationand/or use of organic solvents. Thus, alternative methods ofencapsulation are desired in various technology areas.

SUMMARY OF THE INVENTION

In one aspect, the invention provides a method of making microcapsulescontaining a water-immiscible core material. The method includes:

-   -   (a) forming an emulsion including droplets of a water-immiscible        core material in a first aqueous solution of an amphiphilic        first wall-forming reactant that preferentially accumulates at        the surface of the droplets;    -   (b) forming a second aqueous solution of a second wall-forming        reactant; and    -   (c) combining the emulsion with the second aqueous solution to        permit a reaction between the first and second reactants such        that the resulting reaction product forms an encapsulating wall        surrounding the core material of each emulsion droplet;    -   wherein one of said first and second reactants includes a Lewis        base reactant and the other includes a Lewis acid reactant, and        wherein the reaction product consists of a salt of the Lewis        base-Lewis acid reactant pair.

In another aspect, the invention provides microcapsules including (a) ashell including the salt product of a Lewis acid reactant and a Lewisbase reactant, and (b) contained within the shell, a core materialincluding a vaporizable active agent.

In yet another aspect, the invention provides microcapsules including(a) a shell including the salt product of a Lewis acid reactant and aLewis base reactant, and (b) contained within the shell, a core materialincluding a flavorant.

In still another aspect, the invention provides microcapsules including(a) a shell including the salt product of a Lewis acid reactant and aLewis base reactant, and (b) contained within the shell, a core materialincluding a dye or colorant.

DETAILED DESCRIPTION OF THE INVENTION

Methods of forming microencapsulated materials according to theinvention will now be described in detail, followed by a description ofsuitable materials for use in making the microencapsulated materials andthen a description of methods for isolation and/or use of thesematerials.

Formation of Microcapsules

In one aspect of the invention, a water-immiscible core material, whichmay comprise an active agent, is encapsulated by a process in which thecore material is emulsified in an aqueous solution of a firstwall-forming reactant (one member of a Lewis acid-Lewis base reactantpair) to form dispersed droplets of the core material in the solution.The core material comprises an active agent, an adjuvant, or both, aswill be described in detail later herein.

The reactant in the first mixture must be amphiphilic, i.e. having bothhydrophilic and lipophilic moieties. Without wishing to be bound by anyparticular theory or explanation, it is thought that forces of polarsolvent interaction drive the lipophilic end to solvate into theless-polar interior of the droplet, leaving the hydrophilic moietiessolvated in the aqueous phase and thus causing the reactant topreferentially accumulate at the droplet surface. Thus it is believedthat the reactant in the first mixture tends to rapidly collect at thedroplet-continuous phase interface as the emulsion is formed,stabilizing the emulsion and providing a reaction site for the secondreactant.

The emulsion of core material in the present method may be performedindependent of the wall-forming step, and the first reactant may beintroduced to the continuous aqueous phase before or after emulsion. Itshould be noted that this method differs from potential alternativemethods such as those of the Speaker patents previously cited, in whichthe first wall-forming reactant must be dissolved in the core material,or in a mixture of the core material and a water-immiscible solvent,without the presence of water. Unlike those methods, the present methoddoes not require complete solvation of any wall material in the corephase. Therefore, a wider variety of core materials may be potentiallyencapsulated by the present inventive method. Because the emulsion stepmay be performed independent of the wall-forming step, the presentinventive method affords greater flexibility with regard to the methodsand timing of the emulsion step. Accordingly greater ease ofmanufacturing is provided by the present inventive method Typically theemulsified state of the core material in the first mixture is maintainedby continuous agitation until the second reactant is added andencapsulation is completed. The emulsion may be further stabilized bythe use of an emulsifying, wetting, or surfactant agent such as sodiumlauryl sulfate, or other materials such as polyethylene glycol esters,sorbitol esters, or similar emulsion-promoting materials either in thecontinuous phase, or in the core material prior to dispersion.

The emulsion thus prepared is then combined with a second mixture,containing the second, complementary Lewis acid or Lewis base reactantof the wall-forming reaction pair, which may be but need not beamphiphilic. The reactants are complementary in that one is a Lewis acidand the other is a Lewis base and together they react to form aninsoluble Lewis acid-Lewis base salt. As used herein the word“complementary” refers to Lewis acids and Lewis bases that react to forminsoluble salts. Upon mixing of the emulsion and the second mixture, thesecond reactant reacts with the first, at least a significant portion ofwhich is believed to be situated at the droplet-water interface. Theresulting reaction product, a salt of the Lewis acid Lewis basereactants, precipitates at the droplet interface to form a stabilizedwall and thereby microencapsulates the emulsified droplet containing thewater-immiscible core material. The precipitation reaction isspontaneous and essentially instantaneous upon mixing of the emulsionand the second mixture at 25° C., and requires no additional heating orcuring steps.

Either a Lewis acid reactant or a Lewis base reactant may be used as thefirst wall reactant dissolved in the continuous phase in which the corematerial is dispersed. In many embodiments, the Lewis base reactant isused because many of these compounds are also effective emulsifyingagents. By contrast, aqueous solutions of suitable Lewis acid compoundsmay or may not readily emulsify the core material. In either case, andespecially when a Lewis acid reactant is used, emulsification of thecore material may be facilitated by the addition of a small quantity ofan emulsifying agent, such as almost any common surfactant, for examplesodium lauryl sulfate.

Generally, microdroplets may be formed by almost any physical method ofemulsification. The first reactant is typically dissolved in the aqueousphase prior to such dispersion. However, this is not a requirement, andthe first reactant may also be dissolved or partitioned into the aqueousphase after formation of droplets of the core material. In practice, theemulsion is often conveniently stabilized by the first reactant.Typically the second reactant is conveniently added in the form of aprepared aqueous mixture, usually a solution, but it may instead beintroduced neat and allowed to dissolve or it may be partitioned intothe aqueous phase from some other source, such as a solution of thesecond reactant in a solvent or introduced in some other manner.

An advantage of the encapsulating salt wall of the instant invention isthat it imparts a water-wettable quality to the encapsulated droplet,which may cause the microcapsules to behave as a flocculated suspension,that is to form light, fluffy aggregates. Without wishing to be bound byany particular theory or explanation, it is believed that theflocculated particles aggregate through attractive interaction, but areprevented from caking, or forming dense aggregates, by strong solvationeffects, or wetting of the particles. Regardless of the mechanism, theencapsulating wall appears to affect the suspension in a manner similarto the action of flocculating agents, for example sorbitol esters,commonly used to produce similar effects. Flocculating agents are wellknown in the field of pharmacy, and serve a number of beneficialpurposes in preparations of suspended particles and droplets, includingcontrolled sedimentation volume, resistance to caking or breakinginstability, and easy dispersion, all of which may be exploited toenhance shelf life stability. The flocculant quality of the microcapsulewall in the instant invention may be increased by inclusion ofadditional flocculating agents, resulting in compositions that showbehavior typical of fully flocculated particulate suspensions.

Some or all effects of additional flocculating agents may besubstantially increased by the presence of the capsule wall.Sedimentation volume may be observed to reflect this action. Forinstance when a particularly buoyant core material is emulsified, thedroplets may float to the top, forming a concentrated layer, andpossibly re-merge to form a continuous layer. If a flocculating agent isincorporated into such an emulsion, the droplets still float and form aconcentrated layer, but they are protected from close interaction by asheathing solvation layer that increases the volume of the concentratedlayer, i.e., the sedimentation volume. If the droplets are furtherencapsulated using the instant invention, the sedimentation volume, thefinal volume occupied after settling, typically exceeds that of theemulsion stabilized only by the flocculating agent but lacking themicrocapsule wall. The magnitude of this effect varies according to thespecific materials used, but a ten to fifty percent increase insedimentation volume is commonly observed using encapsulation by theinstant method when compared with equivalent emulsions lacking suchencapsulation.

If the microcapsules contain a core that is liquid at use temperatures,they typically break during application or thereafter, e.g., uponevaporation of the water (if an aqueous carrier is used). On the otherhand, robust microcapsules exhibiting little or no breakage may beproduced if the core contains a substantial fraction of a solidmaterial. Further, liquid microcapsules may also be stabilized againstbreakage by specific carrier formulations. Breaking and non-breakingmicrocapsules will be discussed in more detail later herein.

Shell Wall-Forming Reactants

Substantially any of the wall-forming reactant pairs and adjuvantsdisclosed in the Speaker et al patents, referred to above, may beutilized in the methods of the present invention. The reactant pairscomprise a Lewis acid reactant and a Lewis base reactant. As usedherein, the term “Lewis acid reactant” means a water-solvated ionderived from either a Lewis acid itself or a water-soluble salt of theacid. The term “Lewis base reactant” is analogously defined, whereinwater-soluble salts of Lewis bases are defined herein to also includequaternary ammonium salts. Thus, a wall-forming reactant pair may be acombination of the ions of a Lewis acid and of a Lewis base, whether therespective ions may derive from the dissolved acid or base, or thewater-soluble salt thereof. Unless otherwise specified, or clear fromthe context, mention of the presence of a particular Lewis acid or aparticular Lewis base as a component of a composition will be understoodto encompass either the free acid or base, or the salts of these.

In particular, the Lewis acid reactant may be contributed by any one ora combination of acacia, agar, arabic acid, carboxymethylcellulose,ghatti gum, polyacrylic acid, polyacrylic acid/polyoxyethylenecopolymer, sterculia gum, sodium alginate, sodiumcarboxymethylcellulose, sodium polyacrylate, and sodium polyacrylatecross-linked with polyoxyethylene, alginic acid, pectin or otherpolyuronic acids, or other acidic gums or salts thereof. Generally anyhigh molecular weight or polymeric Lewis acid reactant makes aneffective wall-forming component, and the reactant may comprise one ormore acid groups per molecule. If the Lewis acid reactant is used as thefirst reactant introduced to the dispersed core material, it must beamphiphilic so as to properly associate with both the aqueous phase andthe core material.

Sources of the complementary wall-forming Lewis base reactant includebenzalkonium chloride, cetylpyridinium chloride, hexylamine,hexanediamine, hexamethylrosanilium chloride, piperidine, triethylamine,triethylenediamine, stearylamine, spermine, and tetramethylrosaniliumchloride, or any other similar amine or amine salt capable ofprecipitating the Lewis acid. These representative Lewis bases areamphiphilic and easily support dispersion of many non-aqueous corematerials, and satisfy the requirement of accumulation at the dropletinterface. However, the Lewis base need not be amphiphilic if the corematerial is dispersed in the other aqueous mixture, i.e., the Lewis acidmixture.

Core Materials

Core materials according to the invention may include an active agent,an adjuvant, or both. As used herein, the term “active agent” means amaterial that performs a primary function of the formulation, such asrepelling insects, providing a fragrance, etc. An adjuvant is a materialthat enhances the efficacy of the active agent, for example by slowingits rate of release or by forming a barrier between the active agent anda surface to which it is applied, such as skin. Other forms of activitymodification will be apparent to those of skill in the relevantapplication areas in which the encapsulated materials are used. It willbe appreciated that the distinction between active agent and adjuvantmay be somewhat arbitrary in some situations, and that for example agiven material may be an adjuvant in some applications while in otherapplications it may stand alone, and thus be the active agent. Havingsaid that, materials typically used as active agents will now bediscussed, followed by a discussion of materials typically used asadjuvants.

The core may comprise an insect or arthropod repellent. For example,DEET (i.e. N,N-diethyl-m-toluamide) may be encapsulated and combined ina carrier composition for aerosol or liquid or creamed fluid topicalapplication, or a soap-like bar. DEET is among the most effective agentsused for repelling insects such as mosquitoes and other harmfularthropods such as ticks, and is widely used to prevent their bites andthe exposure to infectious disease that may result. MicroencapsulatedDEET according to the invention has the benefit that the DEET diffusesthrough the microcapsule wall and evaporates at a controlled rate,providing a relatively long-lasting vapor source of the DEET near theskin of a treated individual. Also, it is to be noted that traditionalcompositions may not effectively protect against any undesired effectsthat might result from exposure to DEET. As will be discussed in detailbelow, the use of certain adjuvants with the microencapsulated DEET mayreduce exposure of the underlying skin to the DEET.

A further application of liquid-containing microcapsules, that breakupon drying, is to encapsulate a sunscreen agent or solution thereofwhich is thereby distributed evenly over a skin surface upon drying ofthe film. Octyl-dimethyl para-amino benzoic acid (Padimate O), is acommon liquid sun-protection active ingredient. Padimate O is easilymicroencapsulated using the methods and materials of the instantinvention, and the microcapsules thus formed break upon drying in amanner similar to the silicone oil microcapsules described below. Thesemicrocapsules may be included in a blended product containingadditionally, for example, a microencapsulated insect repellent or afragrance. Another common sunscreen agent,2-hydroxy-4-methoxy-benzophenone (Oxybenzone) is a solid at normalambient temperatures, and cannot be directly encapsulated in the mannerof Padimate O. Oxybenzone may, however, be dissolved in silicone oil oranother suitable water-immiscible oil, and the solution may beencapsulated using the methods described for silicone oil below. Theresultant capsules break open upon drying in a thin film, distributingthe sunscreen agent as desired. Non-breaking capsules containingsunscreen agents may also be produced and may provide a lower risk forskin absorption of the agent. Additionally, the finely dividedencapsulated droplets of sunscreen may provide enhancement of sunprotection by increasing light scattering, thereby reducing the amountof ultraviolet radiation reaching the skin. Flavorants and fragrances(e.g., limonene and cinnamic aldehyde) may also be microencapsulatedaccording to the invention, as will be discussed in detail below.

Other repellents and/or insecticides may also be incorporated insteadof, or in addition to, DEET. These compounds may be similarlyencapsulated using the method and materials of the present invention.For example, the compound 1-methylpropyl2-(2-hydroxyethyl)-1-piperidinecarboxylate (commonly referred to asicaridin or picaridin) is another common lipid-soluble insect repellent,and may be encapsulated using the methods and materials of the instantinvention to provide similar benefit. Permethrin is another commoninsect repellent and insecticide that is similarly lipid-soluble and maybe microencapsulated in a similar manner to provide a topical productexhibiting low dermal absorption. Lipid-soluble colorant agents, such asD&C Red #17 dye and similar materials, are also well-suited toencapsulation by these techniques. Encapsulation of lipid-soluble dyesdissolved in an adjuvant material to produce a microcapsule resistant tobreakage on drying may produce resulting aqueous suspensions, that whenapplied to skin, show strong coloration but show much reduced tendencyto stain the skin even after drying, because the dyes remainencapsulated. Similarly a core material composed of a colored wax may beencapsulated. Such compositions may for example be used aswater-removable cosmetic colorants or face-paints. Conversely,encapsulation of such lipid soluble colorant agents in capsules thatrupture upon drying in a thin film may be useful in providing awater-removable quality prior to drying, but releasing the colorantagents for more permanent coloration once the films dries, as for along-lasting cosmetic colorant or skin-marking agent with a brief periodof easy removability following application. Such breakable microcapsulesmay be produced by using a core material that is liquid at ambienttemperature, for example the core material may be prepared by dissolvinga lipid-soluble colorant such as D&C Red #17 dye in an encapsulableliquid, such as mineral oil. Combinations of microencapsulatedrepellents and cosmetic face-paints may be thus formulated, for exampleto provide a camouflage paint with an intrinsic insect repellentproperty. Alternately, microencapsulated repellents may simply becompounded as an ingredient in a more standard facepaint formulation.

Adjuvants may be selected to perform any of a variety of functions,including to enhance the physical or chemical stability of the corematerial and/or the microcapsules in which the core material isencapsulated, and/or to modify the density of the product and/or tocontrol the release rate or diffusion rate of the active agent throughthe capsular wall and/or to influence the dispersion and/or wetting ofthe core material during or after the encapsulation process and/or tosubsequently affect the carrier phase by diffusing out of the capsule.It is important that any such adjuvant or the combination thereof withother core material not render the core material completely watermiscible, that is, at least some part of the core material must remainwater immiscible. In some embodiments, preferred adjuvants are thosesuch that the core material is liquid at some elevated temperature, atwhich the encapsulation reaction is carried out, but solid at somepre-selected temperature, such as normal ambient temperature (typicallyabout 20° C.). In some cases, it is desired that the core be solid atnormal skin temperature, for instance for some cosmetic applications.Suitable adjuvants may include paraffin wax, fatty alcohols, esters offatty alcohols, glycol ethers of fatty alcohols, triglycerides,polyethylene glycol esters, polyethylene glycol ethers, and fats.

For example, a wax may be combined with DEET in a ratio such that thecombination melts, for example, at a temperature of 50° C., and thiscomprises the core material to be encapsulated. The encapsulationprocess is then carried out above 50° C., at which temperature the corematerial is in a liquid state and conducive to emulsification. As thereaction product is returned to normal ambient temperature, the corematerial solidifies. This imparts physical robustness to themicrocapsule such that it resists deformation, such as experiencedduring drying in a thin film of dispersed microcapsules. The resultingfilm may be fully dried and then re-wet without substantial loss ofcapsule dispersion, because the capsules remain stable and maintain thedroplet integrity. In the absence of a solidifying agent, such thin-filmdrying may in some cases cause deformation and rupture of microcapsules.As adjuvant, the wax further imparts a modified controlled releasecharacteristic as it changes the diffusion rate of the encapsulated DEETout of the capsule. Some adjuvants include solids that melt above thetemperature at which the product is to be used (for example, skintemperature in the case of a product for topical application) so thatthe microcapsule core remains solid in use. This may be useful in caseswhere it is desired that the microcapsules not break, since the presenceof solids in the core (even in the presence also of a liquid) tends tostrengthen the microcapsules against crushing.

In many (but not all) applications, the adjuvant must remain misciblewith the active agent during dispersion to form suitable droplets, andinsufficient miscibility may in some cases be detrimental. For instance,paraffin wax, when used in a DEET-wax combination core, is sufficientlynon-polar that in some core concentrations, water absorbed by DEET oncontact with an aqueous continuous phase, such as with the Lewis acid orLewis base mixture used in encapsulation, causes separation of theparaffin from the DEET to form a separate, third phase. If such amixture of DEET and paraffin wax is dispersed in an aqueous medium, thedroplets formed may not be of uniform composition due to rapid phaseseparation during mixing. For this reason more polar waxy solidsincluding fatty alcohols such as stearyl alcohol, fatty acid ester orfatty alcohol ester waxes such as cetyl esters wax and other materialswith similar polarity, such as polyethylene glycol and polyethyleneglycol ethers, are miscible in a greater range of proportions with DEET,and are less prone to separation due to DEET water absorption, and aretherefore preferred adjuvants in some applications. Other suitableadjuvants include beeswax, carnauba wax, cholesterol, ethyl stearate,isopropyl myristate, isopropyl palmitate, cetostearyl alcohol, myristylalcohol, cetyl alcohol, oleyl alcohol, behenyl alcohol, solidhydrogenated castor and vegetable oils, hard and soft paraffins, andhard fats such as tristearin

Some appropriate adjuvants, for example polyoxyethylene esters ofsorbitol, may also serve an emulsifying and/or flocculation-promotingfunction. Adjuvants of relatively high polarity, such as for examplemixtures of cetearyl alcohol and polyoxyethylene sorbitol esters, may beof use in combination with relatively polar active agents, such asbenzyl alcohol (which is roughly ten times as polar as DEET). Otheradjuvants include fats and glycol ethers of fatty alcohols.

Additional exemplary adjuvants include silicone oils, one example ofwhich is polydimethyl siloxane (PDMS), commonly known as dimethicone.Moderate-viscosity, low volatility silicone oil products such as DowCorning 200 form a smooth, non-toxic film when applied to skin. Siliconeoils are highly non-polar, and are immiscible with DEET and othersimilar compounds of moderate polarity. Skin wetted with silicone oilhas reduced potential to wetting with DEET and other compounds withwhich the silicone oil is immiscible, and silicone oil may thus reduceabsorption of DEET by the skin. Silicone oil is ordinarily difficult toincorporate into traditional water-based formulations, requiring veryfine dispersion and additional dispersing agents and tending to settleand coalesce due to its lower specific gravity relative to water.Silicone oil may however be microencapsulated by the methods andmaterials of the instant invention to provide a highly stabledispersion.

If no solid component is encapsulated along with the silicone oil, andif the formulation is allowed to dry as a thin film, the microcapsuleswill typically break open during drying, releasing the contents onto thesurface to which the film was applied. By combining a suspension of suchsilicone oil microcapsules with a suspension of DEET microcapsules thatinclude a solid core adjuvant, a hybrid preparation is formed. When thehybrid preparation is applied and permitted to dry in a thin film, mostof the DEET microcapsules remain physically intact, providing insectrepellency, while most of the silicone oil microcapsules break open,wetting the surface and the contact area of the DEET microcapsules withsilicone oil. The silicone oil film thus produced further reduces thedegree of skin contact with the microcapsules containing DEET, andreduces the potential for dermal absorption. A silicone oil film of thetype described above may also contribute to protection against, or delayof dermal absorption of, other agents such as plant allergens frompoison oak or other rhus species.

While a homogeneous core is often desirable, there is no requirementthat the all of the core components remain co-miscible during or afterdispersion. In fact the ejection of an additional phase or phases due towetting of one or more core components may be exploited to formmulti-phase encapsulated cores such as encapsulated emulsions, slurries,foams, or any other desired mixture. If the core components are miscibleprior to dispersion, but separate due to wetting effects as does theparaffin-DEET system described above, a multi-phase core may begenerated in-situ, precluding the need for a separate blending step.

Isolation/Use of Microencapsulated Materials

Microencapsulated compounds (such as DEET) may be used and appliedexactly as produced following formation of capsules, but the compositionmay be further improved by mixing the capsules into a carriercomposition such as an aqueous 1% polyethylene glycol solution which mayaid in uniform application of the product, adhesion of the capsules tothe skin surface, and may inhibit dermal absorption of the active agent,and also may impart a desirable “feel” characteristic. Additionally,upon drying of the composition in a film, for instance as applied toskin, the polyethylene glycol is deposited surrounding themicrocapsules, and may stabilize capsules against breakage and modifythe release characteristic in a manner similar to encapsulated adjuvantsdescribed above. In other specific embodiments it may be preferable toconcentrate, dry, or otherwise harvest the microcapsules from theaqueous continuous mixture, and in some cases to re-suspend them in adifferent carrier composition.

The suspended microcapsules as generated by the reaction processdescribed above may then be subjected to harvesting and/or concentrationand/or resuspension, and/or further dispersion into a carriercomposition by any of the following or other means:

a) Direct dispersion of the entire product suspension into a carriercomposition, as by pouring and mixing the entire contents of themicroencapsulation reaction vessel into a volume of a separate carriercomposition, for instance a 1% solution of polyethylene glycol;

b) Concentration (for example by filtration, sedimentation orcentrifugation), of the newly formed microcapsules out of the reactionmixture, and removal of the bulk of the continuous phase to form a massor aggregate of the microcapsules, which may be re-suspended in a newcarrier composition, or rinsed by repeated concentration/resuspensioncycles prior to a final resuspension and drying to a powder state;

c) Partition of the microcapsules from the formative mixture into acarrier composition of a different phase, for instance by bringing thenewly formed microcapsule suspension into contact with a non-aqueousphase such as dichloromethane which may attract them into suspension inthis phase, such that the non-aqueous phase may then be decanted orotherwise removed from contact with the aqueous phase, and themicrocapsules subsequently concentrated as described in (b) above or (d)below;

d) Evaporation (for example by air-drying, freeze-drying, spray-dryingor other means) of the suspending medium followed by re-dispersal of themicrocapsules in the carrier composition, for instance by spraying thenewly formed microcapsule suspension into an rising stream of dry air,droplets containing populations of microcapsules may dry into smallgranules of microencapsulated material that may be subsequentlyresuspended in a new carrier composition;

e) Any other means of effecting the dispersion of the microcapsules intoa carrier composition suitable for an intended use.

Some forms of capsules, particularly those formulated to break openduring drying in a thin film may not be suitable for drying as in b or dabove, but the more robust embodiments of the instant invention readilysurvive even multiple cycles of drying and re-suspension. A suspensionof capsules robust to drying may be fully evaporated to dryness to forma cake or bar that may be used much like a bar of soap, to applymicrocapsules to wet skin. In particular, use of a carrier mixture thatincludes a stearate, for example sodium stearate, with subsequentconcentration by drying, may produce a soap-like bar that may be rubbedagainst wet skin to distribute microcapsules re-suspended as the drycarrier dissolves against the wet skin. In some embodiments of theinvention, the microcapsules may be used in aqueous compositions such aslotions, crèmes, or sprayable compositions, for example for cosmetic,pharmaceutical, or other uses. In some embodiments, the microcapsulesmay be incorporated, along with the associated water in dropletscontaining the microcapsules, into an oil phase to make a water-in-oilemulsion crème or lotion or ointment.

Microcapsules may be further dispersed in a carrier composition designedto permit uniform application of the microcapsules to a surface such asskin, and to adhere the capsules to the surface, and may include otheragents that impart other desirable characteristics. Various materialsmay be included in the carrier fluid to modify the properties of thecarrier itself. Additional carboxymethylcellulose may be added to alterthe rheological properties of the fluid, for instance to inhibitstratification due to microcapsule density. Methylcellulose or otherappropriate materials may be added to contribute a thermal viscositychange or gelation property. Some polyethylene glycols, for instance PEG3350, alter the carrier rheology making the microcapsule suspension moresprayable, and additionally contribute to lowering product freezingpoint as well as to reduce the availability of encapsulated DEET (and/orother active agents) for dermal absorption. Generally, the microcapsulesuspension may be introduced to a wide variety of carrier materialswhich may impart a similarly broad spectrum of additional properties.

Viscous oils, such as medium viscosity mineral oils, constitute one setof carrier materials capable of imparting substantially differentproperties to microencapsulated materials. By use of materials such asSPAN 80 surfactant, well known to aid in production of stablewater-in-oil type emulsions, it is possible to disperse an aqueoussuspension of microcapsules in an oil base. The result of suchdispersion is an oil-based ointment, consisting of a continuous phase ofmineral oil, in which are dispersed fine droplets of the aqueous carrierof the microcapsule suspension, in which are suspended microcapsules.Such a dispersion may be produced using ordinary ointment formulatingmethods, such that microcapsules suspended in tiny aqueous dropletsdispersed in the oil phase are clearly visible under a typicallaboratory light microscope. The resulting ointment provides highresistance to wash-off, and the aqueous carrier droplets serve tocontrol release of the active material. If such an ointment is made withmicrocapsules containing DEET, the release of encapsulated DEET to thecontinuous oil phase is substantially limited by the low (around 1%)solubility of DEET in water. The aqueous phase serves as a bottlenecklayer to concentration-driven diffusion processes, resulting in a verylong-lasting repellent product suitable for use on skin which will besubmerged or repeatedly exposed to water, and may be appropriate for usein repelling water-borne pests.

Another example of a carrier component that may provide added benefit isa water-soluble sunscreen component such as phenylbenzimidazole sulfonicacid, or a skin protectant such as aloe vera extract. Finely dividedzinc oxide or bentonite clay may similarly be incorporated, to providesun-protective or blemish-concealing or other functions. These compoundsand others may be combined with the carrier base material to lendadditional cosmetic or other qualities to the formulation.

A specific additional application involves the reduction of percutaneousabsorption of a fragrance compound or other odorant, e.g., for thetopical delivery of perfumes and cosmetics in a manner similar to itscapability as a repellent. Reduced skin absorption by the same mechanismdiscussed above may be of importance in such applications as well.Similarly, these products and methods may be useful as delivery vehiclesfor fragrance enhancers generally, such as those used to provide acertain odor to commercial products, such as soaps etc. For example,cinnamic aldehyde is a useful and inexpensive compound in widespread usethat provides a floral scent to soaps and other products. Unfortunatelyit is well known that this compound is a dermal sensitizer, generatingallergic reactions in many consumers. For this reason, it is undesirableto use the compound in applications in which it would be directlyapplied to skin. Cinnamic aldehyde is representative of a wide range offragrance compounds, especially aldehydes and alcohols that tend tooxidize to the corresponding aldehydes. Mixing of such microencapsulatedcompounds with microencapsulated adjuvants that are capable of providinga barrier layer on the skin, such as silicone oil as described above,may reduce the availability of the compounds for dermal absorption, andthus reduce the risk of allergic response due to contact with thesecompounds. Inclusion and/or co-encapsulation of preservatives orcompounds protective against oxidation may also prolong the useful shelflife of products containing these compounds.

Microencapsulation with adjuvants chosen to extend release may alsoprovide useful long-duration qualities to highly volatile fragranceagents, such as limonene and other terpenes. Limonene, a staplecitrus-type fragrance in common use, evaporates very rapidly, and theodor is almost undetectable within an hour of application to skin.Limonene may be directly microencapsulated using the methods andmaterials of the instant invention, but without use of an adjuvant, thefragrance is prolonged little. If limonene is microencapsulated in acore-adjuvant formulation, the duration of fragrance can be extended tomore than one hour. Limonene is well solvated by cetyl esters wax,polyethylene glycol and esters of polyethylene glycol, paraffin wax, andsilicone oil, among other materials suitable as core adjuvants.Limonene's volatility is so great, however, that even doubling theduration through use of an adjuvant-enhanced microcapsule, the fragrancestill dissipates rapidly.

A further increase of release duration for limonene (or other activeagents) may be realized through a multiphase encapsulated product, inwhich two (or more) mutually immiscible adjuvants are included. At leastone of the adjuvants is miscible with the active agent. For example,silicone oil and cetyl esters wax (liquid at 50° C.; other fatty acidester or fatty alcohol ester waxes may also be used) are mutuallyimmiscible due to large differences in polarity. If an active agent, forexample, limonene, is introduced into, for example, a mixture ofsilicone oil and liquefied cetyl esters wax, the agent will partitionbetween the two immiscible oil phases. The phases may then beco-dispersed to form an oil-in-oil emulsion, the differences in polaritymaintaining separation of the two oil phases. If this emulsion is thensubsequently dispersed as droplets into an aqueous phase, asuper-emulsion is formed, having droplets of one oil phase dispersed inthe interior of droplets of a second oil phase, which are themselvesdispersed in the aqueous phase. If this mixture is microencapsulated bythe methods of the instant invention, the resulting product is asuspension of microencapsulated solid microspheres of a moderately polarwax which contain additional microdroplets of a non-polar liquid with anagent, such as limonene, partitioned in both oil phases inside themicrocapsule. Other emulsions containing different proportions of polarand non-polar adjuvant ingredients may be similarly formed, exhibitingvarious interior structures, including large single droplets andmultiple small droplets. The additional phase boundaries inside themicrocapsule may provide an enhanced barrier to diffusion, furtherlimiting the evaporation rate. This preparation extends the duration oflimonene fragrance to multiple hours, depending upon what materials areused. Additionally, the strength of the limonene aroma is greatlyreduced at application, since most of the limonene is trapped inside themicrocapsule preparation, and it evaporates slowly over time. A mixtureof microcapsule types having different core compositions, e.g., somerelatively fast-release and some relatively slow release microcapsules,may provide a more even limonene delivery rate for the product. Thisapproach may be used for other active agents as well.

Cinnamic aldehyde is less volatile than limonene, and if a mixture ofcinnamic aldehyde and limonene is directly applied to skin in theabsence of microencapsulated formulations, the limonene scent isinitially overwhelmingly dominant and then rapidly fades. A suitablebalance of fragrance elements occurs only briefly when much of thelimonene has evaporated. By contrast, a mixture of microcapsules ofcinnamic aldehyde including an adjuvant that provides moderatelyextended release with microcapsules of limonene including two immiscibleadjuvants as described above greatly extends the duration of time duringwhich the fragrance elements contribute at comparable levels to theoverall product scent.

Many volatile fragrances may be encapsulated in this manner, accordingto the invention. Other microencapsulated materials may also be employedas flavorants in chewing gums. The flavorants need not be volatile.Other exemplary flavorants include isoamyl acetate, ethyl cinnamate,ethyl propionate, ethyl butanoate, ethyl hexanoate, and similar esters,methyl salicylate, menthol, and capsaicin. In fact, typically anyflavoring or fragrance ingredient that may be dispersed in an oildroplet may be encapsulated using the instant methods and materials.

EXAMPLES Example 1 DEET-Containing Microcapsules

An aqueous solution A of 0.1%, by weight, sodium carboxymethylcellulose(NaCMC, Amend Drug and Chemical Company, Inc., New York, N.Y.) isprepared by pre-wetting 0.1 gram of NaCMC with about 0.5 mL acetone,then adding about 90 mL water. The mixture is heated to 50° C., and thesolution is stirred until all NaCMC is dissolved to form a clear,colorless solution. The acetone rapidly evaporates out of the solution,which is subsequently cooled to room temperature and further dilutedwith sufficient water to reach a total volume of 100 mL.

An aqueous solution B containing 0.05% benzalkonium chloride is preparedby diluting 0.1 mL of 50% concentrated benzalkonium chloride in aqueoussolution (Spectrum Chemical Manufacturing Corporation, Gardena, Calif.)to a final volume of 100 mL.

A 1.0 mL volume of DEET to be encapsulated is introduced to a 10.0 mLvolume of solution B and dispersed by agitation, coacervation, or anyother emulsion-forming mixing techniques known to those skilled in theart. An emulsion is formed, wherein amphiphilic benzalkonium ionspartition to interfacial regions between the DEET droplets and thesurrounding aqueous solution.

A 10.0 mL volume of solution A is added to the emulsion thus formedcontaining dispersed microdroplets of DEET in Solution B. The mixture isfurther agitated to mix the components, exposing the microdroplets tothe complementary wall-forming component. Ideally the emulsion iscontinually agitated during addition of solution A.

Microencapsulation occurs almost instantaneously, and the resultingcomposition is an aqueous suspension of microencapsulated DEET. Thecapsules thus formed are shelf-stable, showing no evidence of Ostwaldripening or coalescence after months of storage. Some stratification ofthe suspension is observable after more than one month, due tomicrocapsule buoyancy, but the microcapsules are readily returned touniform suspension by minor agitation, such as a simple swirl of thecontainer.

Example 2 Sprayable DEET-Based Insect Repellent

A composition is prepared as in example 1, and is further dispersed intoan additional 10 mL of carrier composition of 1% aqueous polyethyleneglycol 3350 or other water-soluble polyethylene glycol to form an easilyapplied, sprayable product possessing a pleasant feel. The polyethyleneglycol helps the microcapsules adhere to the skin, and also helpsprovide desirable feel characteristics. The composition thus applied isdetectable by a light odor of DEET that persists for several hours, butunlike other applications of dissolved or emulsified DEET products, doesnot lend an oily texture to the skin, nor does it obviously mar or stainclothing. Additionally this composition is readily removed by washingwith soap and water, leaving no noticeable odor. A variation of thisExample is to incorporate polyethylene glycol also (or instead) in thecore material prior to encapsulation, so that it subsequently diffusesinto the carrier during or after the encapsulation process. This reducesthe internal capsule volume and/or renders the core material porous,while introducing polyethylene glycol into the carrier.

Stock solutions for use in Examples 3-7 are prepared as follows:

Sodium CMC Solution 1%

-   1. Carboxymethylcellulose Sodium [CMC] (Amend) 1 g per 100 mL    solution desired.-   2. Wet CMC with isopropyl alcohol 70%, sufficient to form loose    slurry.-   3. Stir continuously while slowly adding 80 mL purified water per    100 mL final volume.-   4. Resulting solution will be lumpy and gummy.-   5. Heat to boiling and stir rapidly for 2 hours open to atmosphere,    to volatilize alcohol.-   6. Some film formation at surface may occur.-   7. Add 10 mL purified water, and stir, covered, for an additional 30    minutes to dissolve any film formed.-   8. Dilute to final volume and store in closed container.

Benzalkonium Chloride Solution 1%

-   1. Benzalkonium Chloride Solution 50% (Spectrum) 2 mL (per 100 mL    solution desired).-   2. Stir continuously while adding 80 mL (per 100 mL solution    desired) hot (80° C.) purified water.-   3. Stir continuously while adding hot (80° C.) purified water.-   4. Stir rapidly for 30 minutes open to atmosphere, to volatilize    alcohol (˜5% of base) from 50% base solution.-   5. Dilute to final volume, cool, and store in closed container.

Example 3 DEET-Based Insect Repellent Crème—30% DEET Core Composition:

DEET (Morflex) 150.0 g Cetostearyl alcohol (Cognis) 85.0 g PEG 6000Distearate (Spectrum) 5.0 g DC-200 Silicone (Dow Corning) 5.0 g PEG 3350(Spectrum) 25.0 g SPAN 60 (Spectrum) 5.0 g TWEEN 60 surfactant(Spectrum) 5.0 g

Solution A:

Sodium CMC solution 1% 50.0 g Water 145.0 g

Solution B:

Benzalkonium chloride solution 1% 25.0 g

Method of Preparation:

-   1. In a 500 mL wide-mouth glass container, heat all Core ingredients    except DEET to melting (˜70° C.) and stir to mix completely.    Maintain heat and rapid stirring.-   2. Add DEET to other Core ingredients, and heat and stir until fully    incorporated into a uniform melt solution. (Some cloudiness may be    present.) Increase stirring speed to high magnetic stirrer RPM    (nominal 700 cm/s shear).-   3. Warm Solution A to a similar (˜70° C.) temperature, and slowly    add to stirring Core mixture over about 1 minute. Initially Solution    A will be emulsified in the Core phase as a water-in-oil type    emulsion. As addition of Solution A continues, the emulsion will    undergo inversion to an oil-in-water type, producing finer and more    uniform droplets than possible by directly dispersing Core in    Solution A. Additionally PEG 3350 will partition from the Core phase    into the aqueous phase during dispersion, and is primarily a carrier    phase component.-   4. Slowly add full volume of Solution B, over about 30 seconds    duration, and permit complete mixing by maintaining rapid stirring    for an additional 120 seconds. Discontinue stirring and remove from    heat.-   5. Transfer resulting mixture to a container with a secure screw    cap, and cool, periodically agitating the contents until ambient    temperature is achieved. If agitation during cooling is    insufficient, creaming due to capsule buoyancy can cause    inhomogeneous thickening. An additional warming-cooling cycle can    recover the batch if inhomogeneous thickening is observed.

Example 4 Sprayable DEET-Based Insect Repellent—30% Core Composition:

DEET (Morflex) 150.0 g Myristyl Alcohol (Cognis) 10.0 g Cetyl Esters Wax(Spectrum) 5.0 g PEG 3350 (Spectrum) 20.0 g SPAN 60 (Spectrum) 5.0 gTWEEN 60 surfactant (Spectrum) 5.0 g

Solution A:

Sodium CMC solution 1% 50.0 mL Water 230.0 mL

Solution B:

Benzalkonium chloride solution 1% 25.0 mL

Method of Preparation:

-   1. In a 500 mL wide-mouth glass container, heat all Core ingredients    except DEET to melting (˜70° C.) and stir to mix completely.    Maintain heat and rapid stirring.-   2. Add DEET to other Core ingredients, and heat and stir until fully    incorporated into a uniform melt solution. (Some cloudiness may be    present.) Increase stirring speed to high magnetic stirrer RPM    (nominal 700 cm/s shear).-   3. Warm Solution A to a similar (˜70° C.) temperature, and slowly    add to stirring Core mixture over about 1 minute. Initially Solution    A will be emulsified in the Core phase as a water-in-oil type    emulsion. As addition of Solution A continues, the emulsion will    undergo inversion to an oil-in-water type, producing finer and more    uniform droplets than possible by directly dispersing Core in    Solution A. Additionally PEG 3350 will partition from the Core phase    into the aqueous phase during dispersion, and is primarily a carrier    phase component.-   4. Slowly add full volume of Solution B, over about 30 seconds    duration, and permit complete mixing by maintaining rapid stirring    for an additional 120 seconds. Discontinue stirring and remove from    heat.-   5. Transfer resulting mixture to a container with a secure screw    cap, and cool, periodically agitating the contents until ambient    temperature is achieved. If agitation during cooling is    insufficient, creaming due to capsule buoyancy can cause    inhomogeneous thickening. An additional warming-cooling cycle can    recover the batch if inhomogeneous thickening is observed.

Example 5 DEET-Based Insect Repellent Bar—10% Core Composition:

DEET (Morflex) 100.0 g Cetostearyl Alcohol (Cognis) 50.0 g SPAN 60(Spectrum) 5.0 g TWEEN 60 surfactant (Spectrum) 5.0 g

Solution A:

Sodium CMC solution 1% 225.0 mL Water 450.0 mL

Solution B:

Benzalkonium chloride solution 1% 25.0 mL

Solidifier:

Sodium Stearate 315.0 g

Method of Preparation:

-   1. Prepare microcapsules as for crème formulations, mixing Core and    Solution A, and then adding Solution B, but do not proceed to    cooling step.-   2. Maintain heat and stirring of microcapsule suspension, and slowly    add Solidifier about one tenth at a time, allowing the solids to    fully incorporate and disperse in the stirring mixture.-   3. When all of the Solidifier has been incorporated, the resulting    mixture may be poured into a sheet or mold and permitted to cool.    The mixture sets to a firm soap consistency, and may be packaged    directly, or further dried to increase firmness and decrease weight.

Example 6 Oil-Based DEET Insect Repellent Formulation—10% CoreComposition:

DEET (Morflex) 150.0 mL Cetostearyl alcohol (Cognis) 65.0 g SPAN 60(Spectrum) 5.0 g TWEEN 60 surfactant (Spectrum) 5.0 g

Solution A:

Sodium CMC solution 1% 50.0 mL Water 200.0

Solution B:

Benzalkonium chloride solution 1% 50.0 mL

Oil Carrier:

Mineral Oil (Spectrum) 975.0 g SPAN 80 (Spectrum) 25.0 g

Method of Preparation:

-   1. Prepare microcapsules as for crème formulations, mixing Core and    Solution A, and then adding Solution B.-   2. Stir Oil Carrier ingredients to fully dissolve SPAN 80 in Mineral    Oil.-   3. Slowly add microcapsule suspension to stirring Oil Carrier    solution, allowing water-based microcapsule suspension to disperse    and emulsify into Oil Carrier.-   4. Stir until mixture is uniform in appearance.

Example 7 Limonene Fragrance Formulation—10% Core Phase A:

Limonene (Morflex) 50.0 g DC 200 (Dow Corning) 25.0 g

Core Phase B:

Myristyl Alcohol (Cognis) 120.0 g PEG 3350 (Spectrum) 20.0 g SPAN 60(Spectrum) 5.0 g TWEEN 60 surfactant (Spectrum) 5.0 g

Solution A:

Sodium CMC solution 1% 50.0 mL Water 200.0

Solution B:

Benzalkonium chloride solution 1% 25.0 mL

Method of Preparation:

-   1. In a 500 mL wide-mouth glass container, heat all Core Phase B    ingredients to melting (˜60° C.) and stir to mix completely.    Maintain heat and rapid stirring.-   2. Mix all Core Phase A ingredients completely, and slowly add Core    Phase A ingredients to Core Phase B melt. Core Phase A emulsifies    into Core Phase B melt. The emulsion thus formed is referred to as    Core Emulsion Mixture.-   3. Warm Solution A to a similar (˜70° C.) temperature, and slowly    add to stirring Core Emulsion Mixture over about 1 minute. Initially    Solution A will be emulsified in the Core Emulsion Mixture as a    water-in-oil type emulsion. As addition of Solution A continues, the    emulsion will undergo inversion to an oil-in-water type, producing    finer and more uniform droplets than possible by directly dispersing    Core Emulsion Mixture in Solution A. Additionally PEG 3350 will    partition from the Core Phase B into the aqueous phase during    dispersion, and is primarily a carrier phase component.-   4. Slowly add full volume of Solution B, over about 30 seconds    duration, and permit complete mixing by maintaining rapid stirring    for an additional 120 seconds. Discontinue stirring and remove from    heat.-   5. Transfer resulting mixture to a container with a secure screw    cap, and cool, periodically agitating the contents until ambient    temperature is achieved.

The compositions of this invention may be readily manufactured and arewell suited for use in the applications described herein, as well asothers. The simple mixing steps involved in the methods of theirpreparation make them particularly adaptable to scaling to an industrialcontinuous-flow process. As exemplified herein, such methods may beperformed in the absence of organic solvents, thereby obviatingpotential health and environmental concerns.

Thus an improvement is provided to existing preparations for superficialapplication of active agents, particularly vaporizable ones andespecially ones for which it is desired to minimize contact with and/orabsorption into the underlying surface. Further, the methods andcompositions of this invention tend to preserve the desirable structureof an emulsion through the physical formation of the encapsulating wall,permitting use of specific advantageous carrier compositions withoutloss of commercial shelf life. In particular, the instant invention hasuseful application as a means to provide a useful insect repellentformulation containing DEET as a microencapsulated active component butminimizing actual skin contact with DEET itself, thereby limiting thepotential for dermal transport and absorption.

Although the invention is illustrated and described herein withreference to specific embodiments, it is not intended that the subjoinedclaims be limited to the details shown. Rather, it is expected thatvarious modifications may be made in these details by those skilled inthe art, which modifications may still be within the spirit and scope ofthe claimed subject matter and it is intended that these claims beconstrued accordingly.

1. A method of making microcapsules containing a water-immiscible corematerial comprising: (a) forming an emulsion comprising droplets of awater-immiscible core material in a first aqueous solution of anamphiphilic first wall-forming reactant that preferentially accumulatesat the surface of the droplets; (b) forming a second aqueous solution ofa second wall-forming reactant; and (c) combining the emulsion with thesecond aqueous solution to permit a reaction between the first andsecond reactants such that the resulting reaction product forms anencapsulating wall surrounding the core material of each emulsiondroplet; wherein one of said first and second reactants comprises aLewis base reactant and the other comprises a Lewis acid reactant, andwherein the reaction product consists of a salt of the Lewis base-Lewisacid reactant pair.
 2. A method as recited in claim 1, wherein saidLewis base reactant comprises quaternary ammonium ions.
 3. A method asrecited in claim 1, wherein said Lewis base reactant comprises one ormore compounds from the group consisting of benzalkonium chloride,hexylamine, hexanediamine, hexamethylrosanilium chloride, piperidine,triethylamine, triethylenediamine, spermine, stearylamine, andtetramethylrosanilium chloride.
 4. A method as recited in claim 1,wherein said Lewis acid reactant comprises a carboxylic acid.
 5. Amethod as recited in claim 1, wherein said Lewis acid reactant comprisesa polycarboxylic acid.
 6. A method as recited in claim 1, wherein saidLewis acid reactant comprises one or more compounds from the groupconsisting of acacia, agar, arabic acid, carboxymethylcellulose, ghattigum, polyacrylic acid, polyacrylic acid/polyoxyethylene copolymer,sterculia gum, sodium alginate, sodium carboxymethylcellulose, sodiumpolyacrylate, and sodium polyacrylate cross-linked with polyoxyethylene,alginic acid, and pectin and other polyuronic acids.
 7. A method, asrecited in claim 1, wherein the core material comprises an active agentand an adjuvant miscible therewith.
 8. A method, as recited in claim 7,wherein the core material is solid at normal ambient temperature, andwherein the step of combining the emulsion with the second aqueoussolution is performed at a temperature at which the core material ismolten.
 9. Microcapsules produced in accordance with claim
 1. 10.Microcapsules produced in accordance with claim 1, further comprising acarrier composition.
 11. Microcapsules produced in accordance with claim1, wherein the core material comprises a vaporizable active agent. 12.Microcapsules as recited in claim 11, wherein the core materialcomprises two mutually immiscible, water-immiscible adjuvants, at leastone of which is miscible with the vaporizable active agent. 13.Microcapsules as recited in claim 12, wherein the vaporizable activeagent comprises limonene and the two mutually immiscible,water-immiscible adjuvants are polydimethylsiloxane and a fatty acidester wax or fatty alcohol ester wax.
 14. Microcapsules as recited inclaim 11, wherein the vaporizable active agent is an insect repellent orarthropod pest repellent.
 15. Microcapsules, as recited in claim 11,wherein the vaporizable active agent is DEET.
 16. Microcapsules producedin accordance with claim 8, wherein the adjuvant comprises one or morecompounds selected from the group consisting of paraffin wax, fattyalcohols, esters of fatty alcohols, glycol ethers of fatty alcohols,triglycerides, polyethylene glycol esters, polyethylene glycol ethers,and fats.
 17. Microcapsules produced in accordance with claim 8, whereinthe adjuvant comprises a polyethylene glycol.
 18. Microcapsules producedin accordance with claim 8, wherein the adjuvant comprises a siliconeoil.
 19. Microcapsules comprising (a) a shell comprising the saltproduct of a Lewis acid reactant and a Lewis base reactant, and (b)contained within the shell, a core material comprising a vaporizableactive agent.
 20. Microcapsules, as recited in claim 19, wherein thecore material comprises a material that is solid at a pre-selectedtemperature.
 21. Microcapsules, as recited in claim 20, wherein thematerial is solid at normal ambient temperature.
 22. Microcapsules, asrecited in claim 20, wherein the material is solid at skin temperature.23. Microcapsules as recited in claim 19, wherein the core material isimmiscible with water.
 24. Microcapsules as recited in claim 19, whereinthe core material further comprises an adjuvant miscible with thevaporizable active agent.
 25. Microcapsules as recited in claim 19,wherein the core material further comprises two or more mutuallyimmiscible, water-immiscible adjuvants, at least one of which ismiscible with the vaporizable active agent.
 26. Microcapsules as recitedin claim 25, wherein the vaporizable active agent comprises limonene andtwo mutually immiscible, water-immiscible adjuvants arepolydimethylsiloxane and a fatty acid ester wax or a fatty alcohol esterwax.
 27. Microcapsules as recited in claim 19, wherein the vaporizableactive agent is a fragrance or flavorant.
 28. Microcapsules as recitedin claim 19, wherein the vaporizable active agent is an insect repellentor arthropod pest repellent.
 29. Microcapsules as recited in claim 19,wherein the vaporizable active agent is DEET.
 30. Microcapsules, asrecited in claim 28, wherein the adjuvant modifies the rate at which thevaporizable active agent diffuses through the capsule wall. 31.Microcapsules, as recited in claim 28, wherein the adjuvant comprisesone or more compounds selected from the group consisting of paraffinwax, fatty alcohols, esters of fatty alcohols, glycol ethers of fattyalcohols, triglycerides, polyethylene glycol esters, polyethylene glycolethers, and fats.
 32. Microcapsules, as recited in claim 28, wherein theadjuvant comprises a polyethylene glycol.
 33. Microcapsules, as recitedin claim 28, wherein the adjuvant comprises a silicone oil. 34.Microcapsules as recited in claim 28, further comprising a combinationof microcapsule populations containing different core compositions thatrelease the vaporizable active agent at different rates.
 35. A solid barcomprising microcapsules according to claim 28, wherein themicrocapsules become re-suspended in water when rubbed against wet skin.36. An aqueous composition comprising dispersed microcapsules accordingto claim 19, further comprising a polyethylene glycol dissolved in theaqueous phase.
 37. An aqueous composition comprising dispersedmicrocapsules according to claim 19, further comprising dispersedmicrocapsules containing a silicone oil.
 38. A water-in-oil emulsioncomprising microcapsules according to claim
 19. 39. A lotion, crème, orsprayable composition comprising dispersed microcapsules according toclaim
 19. 40. A method of treating animal skin to cause the skin to berepellent to an insect or arthropod, the method comprising applying tosaid skin a composition including microcapsules as recited in claim 28.41. A method of treating skin to apply an active agent or combination ofagents, the method comprising applying to the skin a compositionincluding microcapsules as recited in claim
 19. 42. Microcapsulescomprising (a) a shell comprising the salt product of a Lewis acidreactant and a Lewis base reactant, and (b) contained within the shell,a core material comprising a flavorant.
 43. Microcapsules comprising (a)a shell comprising the salt product of a Lewis acid reactant and a Lewisbase reactant, and (b) contained within the shell, a core materialcomprising a dye or colorant.